The invention relates to updating image frames on a screen, particularly on screens comprising memory.
Display units are used for example in different computer systems and portable terminals, such as mobile stations, for displaying text and images to the user of the device. A display signal, typically comprising a display data signal and a synchronization signal, is input to the display unit. The display data signal comprises image frames that are distinguished from each other by means of a frame synchronization signal comprised by the synchronization signal. The synchronization signal also comprises a line synchronization signal for determining the separation of the lines comprised by one image frame.
Image frames are displayed to the user of a device by means of a display screen that receives image frames at an encoding rate that thus determines the number of image frames received in a time unit. Image frames are updated on the screen at a frequency called screen refresh rate. For each display unit, the manufacturer typically defines a target refresh rate that is determined according to the properties of the screen and the interfaces associated therewith. Since the costs of a display unit typically have to be kept within given limits, the above properties also set a maximum value for the target refresh rate. This may lead to a situation where the refresh rate to be used is lower than the encoding rate at which the display unit receives image frames. In this case the display unit has to convert the frame frequency by adapting the image frames received at the encoding rate to be displayed on the screen at the refresh rate.
Modern display units, particularly suitable for displaying moving image, such as video or various games, typically comprise memory for buffering image frames before the image frames are displayed on the screen. One image frame can typically be stored in the available buffer memory, allowing the frame frequency to be converted by reading the data comprised by the stored image frame and updating it to the screen at the refresh rate used.
The problem in the above arrangement is the limited size of the buffer memory available in display units, which easily causes what is known as tearing to the image displayed on the screen, i.e. one part of the image displayed on the screen is composed of one image frame and another part of another image frame. On the screen this is visible as flashing lines or breaking in a laterally moving object. Tearing results from a new image frame being stored in the buffer memory before the previous image frame is entirely updated on the screen.
To avoid tearing, the display unit could use a larger buffer memory having sufficient capacity for storing for example two image frames, ensuring that a new image frame would not be stored onto a image frame to be updated to the screen and still residing in the memory before the previous image frame is entirely updated to the screen. However, a larger buffer memory causes extra costs and the space it requires becomes a problem particularly in small devices, such as mobile stations. This means that a larger buffer memory would have to be integrated into the same IC circuit together with other signal-generating components, which may be impossible to implement sufficiently cost-effectively.
Various software-based solutions are also known for converting the frame frequency, in which the control system of the display unit converts image frames received at the encoding rate into the refresh rate in accordance with instructions given by the microprocessor. One such method is known from U.S. Pat. No. 6,054,980. However, the drawback of such methods is consumption of processing power and memory.
The object of the invention is thus to provide a method and an apparatus implementing the method so as to avoid the above problems. The objects of the invention are achieved with a method, a system and a mobile station characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on coupling synchronization signals from an image frame to be displayed on the screen also to the control system, allowing the control system to be timed on the basis of synchronization signals, particularly the pulses comprised thereby, to input a new image frame to the frame buffer such that the new image frame is not stored onto a image frame being updated to the screen. The synchronization signals are used to control the storage of a new image frame in the frame buffer such that each line of the screen is updated before the frame information to be created on the corresponding line of the following image frame is stored in the frame buffer.
In accordance with a preferred embodiment, feedback synchronization signals are combined to form one signal, allowing the control system to interpret both synchronization signals correctly from said one signal. The synchronization signals are preferably combined to form one signal by means of a logic OR or XOR (Exclusive OR) operation.
The advantage of the method and system of the invention is the avoidance of tearing and the ability to ensure the timely update of image frames to the screen. A further advantage is that updating the frame buffer on the basis of synchronization signals does not consume the processing power or memory of the control system since no special program code is needed to carry out the conversion of the frame frequency. Still another advantage is the ability to utilize the existing frame buffer of the display unit; no larger buffer memory is needed for the simultaneous storage of several image frames. A further advantage is that the feedback of synchronization signals according to the invention is extremely simple to implement.